Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

java/google/registry/flows/domain/DomainFlowUtils.java

@@ -509,9 +509,7 @@ public class DomainFlowUtils {
     // If the resultant autorenew poll message would have no poll messages to deliver, then just
     // delete it. Otherwise save it with the new end time.
     if (isAtOrAfter(updatedAutorenewPollMessage.getEventTime(), newEndTime)) {
-      if (autorenewPollMessage.isPresent()) {
-        ofy().delete().entity(autorenewPollMessage.get());
-      }
+      autorenewPollMessage.ifPresent(autorenew -> ofy().delete().entity(autorenew));
     } else {
       ofy().save().entity(updatedAutorenewPollMessage);
     }

java/google/registry/flows/domain/DomainTransferApproveFlow.java

@@ -207,9 +207,7 @@ public final class DomainTransferApproveFlow implements TransactionalFlow {
         autorenewEvent,
         gainingClientPollMessage,
         gainingClientAutorenewPollMessage);
-    if (billingEvent.isPresent()) {
-      entitiesToSave.add(billingEvent.get());
-    }
+    billingEvent.ifPresent(entitiesToSave::add);
     ofy().save().entities(entitiesToSave.build());
     // Delete the billing event and poll messages that were written in case the transfer would have
     // been implicitly server approved.

java/google/registry/flows/domain/DomainTransferRequestFlow.java

@@ -197,9 +197,7 @@ public final class DomainTransferRequestFlow implements TransactionalFlow {
             existingDomain,
             trid,
             gainingClientId,
-            (feesAndCredits.isPresent())
-                ? Optional.of(feesAndCredits.get().getTotalCost())
-                : Optional.empty(),
+            feesAndCredits.map(FeesAndCredits::getTotalCost),
             now);
     // Create the transfer data that represents the pending transfer.
     TransferData pendingTransferData =

java/google/registry/flows/domain/DomainUpdateFlow.java

@@ -193,9 +193,7 @@ public final class DomainUpdateFlow implements TransactionalFlow {
     entitiesToSave.add(newDomain, historyEntry);
     Optional<BillingEvent.OneTime> statusUpdateBillingEvent =
         createBillingEventForStatusUpdates(existingDomain, newDomain, historyEntry, now);
-    if (statusUpdateBillingEvent.isPresent()) {
-      entitiesToSave.add(statusUpdateBillingEvent.get());
-    }
+    statusUpdateBillingEvent.ifPresent(entitiesToSave::add);
     EntityChanges entityChanges =
         customLogic.beforeSave(
             BeforeSaveParameters.newBuilder()